This invention relates to polytetrafluoroethylene compositions and the fabrication of articles therefrom.
Polytetrafluoroethylene (PTFE) is known for its non melt-fabricability, i.e. it has a melt viscosity which is so high, e.g. at least 108 Paxc2x7s at 380xc2x0 C., that it does not flow in the molten condition. Consequently, the PTFE cannot be fabricated by the usual melt-fabrication process of extrusion, including injection molding. Instead, fabrication processes have been developed which do not involve melt flow. For the fine powder type of PTFE, wherein the PTFE is fibrillatible when subjected to shear, the PTFE is paste extruded, which is a low temperature (less than 75xc2x0 C.) extrusion process carried out on a mixture of the fine powder type of PTFE and lubricating oil. The granular type of PTFE is fabricated by compression molding, which involves filling the mold with cold granular PTFE, and then subjecting it to compression by a ram and heating to form the molded article. Among the problems associated with paste extrusion is the need to remove the lubricant after paste extrusion. Compression molding of granular PTFE is limited to simple shapes. Neither type of PTFE is injection moldable, whereby when intricate shapes of the PTFE are desired, they have to be machined from compression molded blocks of PTFE.
The present invention provides polytetrafluoroethylene compositions which are melt fabricable by extrusion and thus by injection molding. The composition comprises at least 50 wt % polytetrafluoroethylene, at least 20 wt % of which has a melt viscosity of at least 108 Paxc2x7s, and the remainder polyaryleneetherketone, to total 100 wt % of the combination of these two resins.
When extruded in the molten form, the composition has a matrix of PTFE and the polyaryleneetherketone is dispersed therein as fine particles (domains) which solidify from the melt. Thus, molded articles of the melt-fabricated composition have much of the characteristics of PTFE, e.g. high service temperature, chemical inertness, and lubricity (low friction). As will be explained hereinafter, the composition can contain at least 90 wt % of the PTFE, which furthers the attributes of the molded article being like pure PTFE.
The polyaryleneetherketone is a well known family of resins which includes polyetherketone having the repeat unit 
polyetheretherketone having the repeat unit 
and polyetherketoneketone having the repeat unit 
wherein the arylene group between the ketone groups of the polyetherketoneketone is typically a mixture of para- and meta-arylene groups, derived for example from isophthalyl and terephthalyl halides, present in the repeat units making up the resin. These resins are commonly known as PEK, PEEK, and PEKK, respectively, and are disclosed in one or more of the following U.S. Pat. Nos. 3,065,205, 3,441,538, 3,442,857, 5,357,040, 5,131,827, 4,578,427, 3,516,966, 4,704,448, and 4,816,556. The polyaryletherketones typically have a melting point which is at least 300xc2x0 C. and high molecular weight, high strength and high modulus characteristic of engineering plastics. The inherent viscosity of the resin can for example be at least 0.4 measured on a 0.5 wt % solution in concentrated sulfuric acid at 30xc2x0 C. PEKK is preferred because it can have low melt viscosity and slow crystallization, which facilitates mixing of the resin with the PTFE and providing melt flowability to the resultant composition. PEKK typically contains units derived from both terephthalyl (T) and isophthphalyl (I) halide in a ratio of 90:10 to 30:70, and more typically 80:20 to 60:40. As the proportion of T units decrease and I units increase, the crystallinity of the PEKK diminishes, until at 60:40, the PEKK crystallizes so slowly that it resembles an amorphous polymer except that it will exhibit a melting point. The present invention is applicable to all of the polyaryleneetherketone resins, i.e. they all impart melt-fabricability to the PTFE.
The PTFE is also a well known resin. The fine powder type is preferred and this type is prepared by aqueous dispersion polymerization. The resultant PTFE fine powder type is a fibrillatible resin. A non-fibrillatible, non-melt-fabricable PTFE resin, which is also prepared by aqueous dispersion polymerization as disclosed in U.S. Pat. No. 4,952,636 can also be used in the present invention. The PTFE can be homopolymer of tetrafluoroethylene or can be modified homopolymer, i.e. containing a small amount, e.g. less than 0.5 mol %, of copolymerized monomer which improves the film forming property of the resin, such as disclosed in U.S. Pat. No. 3,142,665. The PTFE will generally have a melt viscosity of at least 108 Paxc2x7s at 380xc2x0 C.
The PTFE and PEKK components can be melt blended as part of the extrusion process or can be premixed, followed by melt blending at temperature at which at least the polyaryleneetherketone is molten. In the case of injection molding, preferably the composition is first melt blended, using for example a twin screw extruder equipped with high shear screws, and formed into molding granules (pellets), followed by injection molding, which procedure promotes uniformity in the dispersion of polyaryleneetherketone domains within the PTFE matrix. Generally, the melt blending temperature will be at least 350xc2x0 C. Under this condition, the polyaryleneetherketone fluidizes the composition and becomes uniformly dispersed as fine molten domains in the PTFE component, enabling the composition to be melt extruded. The resultant extrudate can be the final molded article, such as in the case of an injection molded article or an extruded tube, sheet or coating, or can be chopped into molding granules for subsequent melt processing into the article desired. The presence of the polyaryleneetherketone component as discrete particles (domains) within the PTFE matrix indicates the incompatibility between the two resins. Nevertheless, articles molded from compositions of the present invention have good properties
An amount of the polyaryleneetherketone which is effective to impart melt-extrudability to the composition while maintaining the PTFE as the matrix phase when molded is present in the composition. When the PTFE is the fine powder type, as little as about 20 wt % of the composition can be the polyaryleneetherketone is needed to impart melt fabricability to the resultant composition, whereby the composition will contain 20 to 50 wt % of the polyaryleneetherketone, preferably 20 to 30 wt %. The wt % of the components are based on the total weight of the PTFE and polyaryletherketone components unless otherwise indicated herein.
It has also been discovered that the amount of polyaryleneetherketone in the composition can be reduced while retaining melt fabricability. The addition of PTFE micropowder to the composition enables this result to be achieved such that as little as about 4 wt % of the polyaryletherketone is all that is necessary for imparting melt extrudability to the composition, the 4 wt % being based on the combined weight of the PTFE and polyaryletherketone components. In this embodiment, the PTFE component comprises both the high melt viscosity (non-melt-fabricable) PTFE and the PTFE micropowder. The PTFE micropowder is not merely a fine powder of PTFE. Instead, PTFE micropowder is a tetrafluoroethylene homopolymer or modified homopolymer which has a considerably lower molecular weight than the normal high melt viscosity PTFE, e.g. the PTFE fine powder described above, which enables the micropowder by itself to be melt flowable, the melt viscosity of the micropowder being 50 to 1xc3x97105 Paxc2x7s as measured at 372xc2x0 C. in accordance with the procedure of ASTM D-1239-52T, modified as disclosed in U.S. Pat. No. 4,380,618. Preferably the melt viscosity of the PTFE micropowder is 100 to 1xc3x97104 Paxc2x7s at 372xc2x0 C. PTFE micropowder is described further in Kirk-Othmer, The Encyclopedia of Chemical Technology, 4th Ed., pub. by John Wiley and Sons (1994) on pp 637-639 of Vol. 11, and in the article H. -J Hendriock, xe2x80x9cPTFE Micropowdersxe2x80x9d, Kunstoffe German Plastics, 76, pp. 920-926 (1986). These publications describe the micropowder as being obtained by polymerization or by irradiation degradation of the high molecular weight (high melt viscosity) PTFE. Polymerization directly to the micropowder is disclosed for example in PCT WO 95/23829, wherein the micropowder is referred to as low melt viscosity PTFE. Although the PTFE micropowder is melt flowable, it is not melt fabricable by itself because the resultant product has no practical strength due to the low molecular weight of the PTFE micropowder. Thus, the beading obtained in the melt flow rate test from which the melt viscosity is determined is brittle such that it breaks upon the slightest flexure.
The PTFE micropowder when used, is preferably present in the composition in an amount which is effective to enable the amount of polyaryleneetherketone needed for melt fabricability of the PTFE composition to be reduced by at least 40%. Thus, with the presence of the micropowder, as little as 4 or 5 wt % of the polyaryleneetherketone may be all that is needed for melt fabricability of the composition by melt extrusion. At least about 10 wt % of the polyaryleneetherketone may be desired, to increase fluidity in the melt, for injection molding. When the PTFE is the fine powder type, the composition may contain 4 to 10 wt % polyaryleneetherketone and the remainder being the PTFE fine powder and the PTFE micropowder, the amount of the micropowder in each case being effective to make the composition melt-fabricable. When the PTFE micropowder is used in the composition its amount, will generally be from 20 to 80 wt % based on the combined weight of the micropowder and the PTFE (high melt viscosity), preferably 30 to 75 wt %, whereby at least 20 wt % of the high melt viscosity PTFE, based on total PTFE content, is present, preferably at least 25 wt %. Of course, the entire PTFE content of the PTFE/polyaryleneetherketone blend can be the high molecular weight PTFE.
The combination of the high melt viscosity PTFE and PTFE micropowder provides a fabricated article in which the two types of PTFE are an indistinguishable blend of PTFE resin forming the matrix for the dispersion of polyaryleneetherketone therein. The PTFE resin is highly thermally stable and strong, not being significantly affected by the presence of the polyaryleneetherketone and PTFE micropowder fraction of the PTFE resin.
The compositions of the present invention can contain other ingredients such as pigments for coloring the composition or fillers, such as mica, glass, carbon, or aramid, in fibrous or other particulate form. The polyaryleneetherketone aids in dispersing the filler in the PTFE matrix. When filler is present, the amount present will be 0.5 to 30 wt % based on the combined weight of PTFE and filler.
The melt extrusion of the compositions of the present invention can be carried out in the same manner as extrusion of melt-fabricable fluoropolymers such as TEFLON(copyright) PFA fluoropolymer resin (DuPont) into a wide variety of shapes, such as sheets, tubing, and coated substrate, such as insulation on wire. Similarly, injection molding can be carried out in the same way as injection molding of the PFA fluoropolymer, e.g. at temperatures of 330 to 370xc2x0 C. PTFE fine powder is usually sintered after non-melt fabrication, except in the case of tape made from paste extruded fine powder. In the melt fabrication of compositions of the present invention, just the melt exposure in the fabrication process is all that is necessary to provide the molded article with the integrity desired. Subsequent sintering can be used if further strengthening is desired.
In the following Examples, blends of polyaryleneetherketone and PTFE are melt-fabricated into tensile test bars (xe2x80x9cdog bonexe2x80x9d shape, 8-xc2xd in (21.6 cm) long, 0.5 in (1.27 cm) wide in the test region, and 0.125 in (0.32 cm) thick) using a 6 oz. (170 g) injection molding machine having the following temperature profile in the screw/ram chamber: rear=370xc2x0, center=380xc2x0 C., and front=380xc2x0 C. The screw speed was 120 rpm and the back pressure was 50 psi (0.35 Mpa). The mold temperature was about 110xc2x0 C., and the mold cycle was 20 sec/20 sec. The PEKK resin used in the Examples is made by the process of U.S. Pat. No. 4,816,556 (Gay et al.).